The application of a coating to a substrate is required in a wide variety of engineering applications, including thermal or environmental protection, improved wear resistance, altered optical or electronic properties, decorative, biocompatibility, etc. In each of these cases, the ability to deposit compositionally controlled coatings efficiently, uniformly, at a high rate, with a high part throughput, and in a cost effective manner is highly desired. Electron beam-physical vapor deposition (EB-PVD) is a widely used method for the high-rate production of atomic and molecular vapor (metal or ceramic) for vapor deposition of a coating. During EB-PVD, vapor is transported to a substrate under high vacuum (10−4-10−8 Torr) conditions where it condenses on surfaces that are in the line-of-sight of the vapor flux source. This requires the use of complicated translation and rotation systems to deposit a uniform and conformal (non line of sight) coating onto ligament shaped structures.
Electron beam-physical vapor deposition (EB-PVD) of metal and ceramic coatings can be quite costly to apply due to high equipment cost, low deposition efficiencies and relatively low deposition rates. The high equipment costs of EB-PVD are a result of the high vacuum environment, which is necessary during deposition (typically below 10−6 Torr), the high cost of high power electron beam guns and the sophisticated component manipulation systems needed to achieve uniform and conformal (non line-of-sight) coating on non-planar substrates. The operating pressure defines the vacuum pump requirements with lower pressures generally needing more expensive pumps. The low deposition rate and low materials utilization efficiency (MUE) of EB-PVD is related to the distribution of vapor flux as it leaves the evaporated source. Generally, the vapor flux spreads out from the source with a distribution described by a cosn θ function (where n=2, 3, 4 or more). When relatively long source to substrate distances are required the deposition efficiency is dramatically reduced.
As stated above, the ability to uniformly deposit ceramic or metallic coatings onto structural fibers and wires (10 to 1000 μm in diameter) is desirable for a number of applications. They include the deposition of metal on fibers to create metal matrix composites [1-5], deposition of coatings having low shear resistance and good thermochemical stability on the fibers used in ceramic matrix composites [6-11] and the deposition of metals on sacrificial fiber templates to create hollow fibers [12,13]. More generally, vapor deposition approaches which allow the creation of conformal coatings on a variety of non-planar substrates is also of interest. For example, the deposition of reaction inhibiting coatings into the vias and trenches used for microelectronics [14], the growth of coatings on the ligaments of stochastic foam structures [15] and various coatings on medical stents [16, 17] are all growing in importance.
Several options for creating coatings of this type exist. These include chemical vapor deposition (CVD) [1,18], electroplating processes [7,18] and physical vapor deposition (PVD) approaches (such as electron beam evaporation [19] or inverted cylindrical magnatron sputtering [20]). However, despite the many needs, the advancement of processing approaches for these applications above are limited by several factors. Namely, the inability to uniformly coat such substrates without sophisticated substrate translation and rotation capabilities the inability to deposit metal, alloys and ceramics with the same process and low deposition rates which often limit high volume throughputs.
In CVD, uniform coating thicknesses are readily produced in some cases. However, the deposition rates can be low and the process often requires the use of toxic (and expensive) precursor materials [18]. The deposition of multicomponent alloys can also be challenging. Electroplating can provide uniform coating over the surface of complex shaped parts. Although useful for depositing elemental layers, this process is less suitable for the creation of alloy or ceramic coatings [18].
In PVD approaches vapor atoms are created in high vacuum and deposited onto a substrate. One method to created vapor atoms is sputtering. A wide variety of materials can be deposited, but deposition rates are low [3]. The high vacuums employed in these techniques result in few collisions with the background gas resulting in “line-of-sight” coating. Thus, substrate manipulation is required to achieve coating uniformity. Higher deposition rates require more energetic/higher density plasma sputtering (e.g. magnatrons). But even here rates are only about 0.3 μm/minute.
Atomic vapor can be more rapidly created using electron beam evaporation approaches. However, the materials utilization efficiency (MUE) of electron beam physical vapor deposition is often low (especially for the case of very small substrates such as fibers or wires). This is due to the vapor spreading out from the source with a flux distribution described by a cosn θ function (where n=2-4 or more) [21]. When a relatively long source to substrate distances is required, the deposition efficiency can be low and the deposition rate limited. The high vacuum environments required for the creation of electron beam (approximately 10−4 Pa) also lead to line of sight (LOS) coating.
Low deposition efficiencies result from flux spreading beyond the periphery of the substrate (which is exacerbated by small ligament shaped substrates) and non line-of-sight deposition. One approach to reduce the spread of the flux exploits entrainment of the vapor on a controllable inert (e.g. helium or argon) carrier gas flow. Such an approach is used in electron beam-directed vapor deposition (EB-DVD), details of the EB-DVD process can be found in co-assigned U.S. Pat. No. 5,534,314, issued Jul. 9, 1996, entitled “Directed Vapor Deposition of Electron Beam Evaporant,” of which is hereby incorporated by reference herein in its entirety. In this approach, the combination of a continuously operating electron beam gun (modified to function in a low vacuum environment) and an inert carrier gas jet. In this system, the vapor plume is intersected with a rarefied trans- or supersonic inert gas jet, to entrain the evaporated flux in a non-reacting gas flow and transport it to a substrate under low vacuum (10−2-101 Torr) conditions. Deposition of the atomistic flux then occurs by gas phase scattering from the streamlines of the flow and is deposited onto the substrate at high rates and with high materials utilization efficiency.
Some illustrative examples of deposition systems and methods are provided in the following applications and patents and are co-assigned to the present assignee and are hereby incorporated by reference herein in their entirety: 1) U.S. Pat. No. 6,478,931, issued Nov. 12, 2002, entitled “Apparatus and Method for Intra-layer Modulation of the Material Deposition and Assist Beam and the Multilayer Structure Produced There from,” and corresponding divisional U.S. application Ser. No. 10/246,018, filed Sep. 18, 2002, entitled “Apparatus and Method for Intra-layer Modulation of the Material Deposition and Assist Beam and the Multilayer Structure Produced There from;” and 2) PCT International Application No. PCT/US01/16693, filed May 23, 2001, entitled “Process and Apparatus for the Plasma Activated Deposition in a Vacuum,” and corresponding U.S. application Ser. No. 10/297,347, filed Nov. 21, 2002; 3) PCT International Application No. PCT/US02/13639, filed Apr. 30, 2002, entitled “Method and Apparatus for Efficient Application of Substrate Coating;” and 4) PCT International Application No. PCT/US02/28654, filed Sep. 10, 2002, entitled “Method and Apparatus for Application of Metallic Alloy Coatings,” and these applications and patents are hereby incorporated by reference herein in their entirety.